A motor is a machine that converts one form of energy to another form of energy. An electric motor converts electrical energy into mechanical energy or, occasionally, the reverse. By way of example, electrical energy supplied by a power supply to an electric motor has been used to accelerate, maintain a speed of, and/or decelerate a rotating shaft of the motor. The rotating shaft may be used for various applications such as rapidly decelerating a moving object having significant stored inertial energy or a heavy load that is being lowered against gravity and has significant stored potential energy. During active motor control, the motor may behave as a generator, and stored or potential energy from the motor may be converted back into electrical energy (regenerated energy) that is absorbed by the power supply.
A motor drive is the device that manages the application of electricity from a power supply to a motor in such a way as to achieve a desired motion. A conventional motor drive behaves like a bidirectional energy pump, transferring energy from the power supply into the motor to cause acceleration and from the motor to the power supply to cause deceleration. It does this irrespective of the relative voltages at the power supply and the motor. Generally, the power supply and the motor drive have a limiting maximum voltage. Exceeding this limit may result in damage to the power supply and/or the drive. With a power supply that is predominantly capacitive, the power supply voltage has a tendency to increase as the power supply absorbs regenerated energy. Unless somehow constrained, the voltage can increase above the limiting maximum voltage due to absorbed regenerated energy.
One approach to limiting the increase in power supply voltage during motor regeneration is to include a mechanism that opens the electrical path between the power supply or drive and the motor when the power supply voltage reaches a preset overvoltage threshold. This protects the power supply and drive by interrupting the regeneration process before power supply voltage can reach a damaging level, but it also results in a cessation of active motor control (i.e., the motor drive no longer controls the motor), and the motor coasts until the power supply voltage decays to an acceptable level where active motor control can resume. Generally, the loss of active motor control is undesirable, and repetition of this cycle may cause a chugging behavior as the drive enables and disables. Another approach would be to increase system mechanical losses. Still another approach would be to use a mechanical brake.
Yet another approach includes using a regenerative energy dissipator. With this approach, when power supply voltage increases and approaches the overvoltage threshold due to the absorption of motor regenerated energy, a large resistor is switched into the circuit across the power supply so as to partially discharge the bulk capacitance and limit the voltage increase to acceptable levels. The regenerative energy dissipator removes excess energy from the power supply and dissipates it as heat in a resistor. With sufficient regenerative energy dissipation, motor control can continue without limit, even during rapid deceleration or lowering of heavy objects, because the power supply voltage is not allowed to reach damaging levels. Regenerative energy dissipators work well to manage the energy recovered from the motor. However, they come at the expense of added circuit and system complexity, size, and cost.
This added expense may be hard to justify in smaller and less expensive motor drive products. Furthermore, some mechanical systems using motor drives may not need to manage large amounts of regenerated energy, for example, where deceleration rates are limited and friction losses significant so that only a modest power needs to be removed from the motor via the drive in order to achieve acceptable deceleration performance. Also, the power supply and drive may be designed with a relatively large tolerance for voltage and with a relatively large capacitance so that the impact of regenerated energy is moderated. In such applications, the full capability of a regenerative energy dissipator is overkill given the relatively small amount of regenerated energy that needs to be managed.